As you have noted the quicker sticks will fall back to the lower frequency. And as wit said, why cripple your 800 MHz sticks by keeping the 667 MHz ones in your PC? Go for 800 MHz if you haven't already purchased the new ones.
This article Memory Scaling on Haswell is very detailed and you don't need to read all of it, but the main points you should take away are the 6 important bullets for memory performance (on the front page):
Amount of memory
Number of sticks of memory
Placement of those sticks in the motherboard
The MHz of the memory
If XMP/AMP is enabled
The subtimings of the memory
Clearly you are being held back by the first point - amount of memory. You will fix that with your purchase, and as long as you make sure they're placed correctly and running in Dual Channel you will reach point #4, which is the frequency. Higher is better; make your own decision based on how badly you want the extra 133 MHz and if it fits within your budget.